1. Field of the Invention
The present invention relates to a receiver, a transmitter-receiver, and a communication method, and is suitably applied to, for example, a radio communication system such as a portable telephone system.
2. Description of the Related Art
In a radio communication system, including so-called cellular system, an area for providing communication services is divided into cells of prescribed sizes: in each of the cells a base station as a fixed radio station is located, and a portable telephone serving as a mobile radio station radio-communicates with the base station in the cell in which the telephone exists. Various systems are proposed for the communication systems between a portable telephone and a base station, and a typical one is a time division multiple connection system referred to as a time division multiple access (TDMA) system.
As shown in FIGS. 1A and 1B, for example, in the TDMA system, a predetermined frequency channel is temporally partitioned into frames F0, F1, etc. of a predetermined time width, each of the frames is divided into time slots TS0 to TS3 of a predetermined time width, and a transmission signal is transmitted by using the frequency channel for the timing of the time slot TS0 assigned to the local station. A system of plural communication (so-called multiplex communication) is realized by using the same frequency channel for the efficient use of frequencies. In the following description, the time slot TS0 assigned for transmission is referred to as a transmission slot TX, and the data block (that is, information unit) to be transmitted by one transmission slot TX is referred to as a slot.
A transmitter and a receiver of a radio communication system for performing transmission and reception by using the TDMA system are described below with reference to FIGS. 2 and 3. The transmitter and the receiver, for example, as shown in FIGS. 2 and 3 are mounted on a portable telephone and a base station of a portable telephone system, and used for the communication from the portable telephone to the base station (so-called up-link communication) and the communication from the base station to the portable telephone (so-called down-link communication).
As shown in FIG. 2, a transmitter 1 comprises a convolution coding circuit 2, an interleaving buffer 3, a slotting circuit 4, a modulation circuit 5, a pilot-symbol addition circuit 6, a transmission circuit 7 and an antenna 8. First, an information bit series S1 serving as transmission data is inputted to the convolution coding circuit 2.
The convolution coding circuit 2, which comprises shift registers of a predetermined number of stages and exclusive OR circuits, applies convolution-coding to the inputted information bit series S1, and outputs the resulting coded bit series S2 to the interleaving buffer 3. The interleaving buffer 3 stores the coded bit series S2 in its internal storage area successively. When the coded bit series S2 are stored in the entire storage area (that is, when a desired volume of coded bit series S2 is stored), the buffer 3 re-sequences the order in the coded bit series S2 at random (the re-sequencing of the order is hereafter referred to as interleaving) and outputs coded bit series S3 obtained by interleaving the coded bit series S2 to the slotting circuit 4. In this connection, the interleaving buffer 3 has a storage capacity for a plurality of slots so that coded bit series is dispersed to a plurality of transmission slots TX.
The slotting circuit 4 partitions the coded bit series S3 for every predetermined number of bits so as to assign the coded bit series S3 to the transmission slots TX and successively outputs coded bit groups S4 obtained by assigning the coded bit series S3 to the transmission slots TX to the modulation circuit 5. The modulation circuit 5 applies predetermined modulation (e.g. synchronous-detection-based modulation such as QPSK) to each of supplied coded bit groups S4 and outputs the resulting information symbol groups S5 to the pilot symbol addition circuit 6.
As shown in FIG. 4, the pilot symbol addition circuit 6 adds pilot symbols P to the head position of each symbol group (that is, the head of information symbols I) of the information symbol groups S5 partitioned correspondingly to the transmission slots TX as headers and outputs the resulting transmission symbol groups S6 to the transmission circuit 7. In this connection, the pilot symbols P added in this case are symbols of patterns previously known to the receiver side, and the receiver side estimates transmission-line characteristics (e.g. the state of fading) in accordance with the pilot symbols P.
The transmission circuit 7 applies filtering to the pilot-symbol-added transmission symbol groups S6 in sequence and applies digital-analog conversion to the resulting groups S6 to generate a transmission signal. Then, the transmission circuit 7 generates a transmission signal S7 of a predetermined frequency channel by applying frequency conversion to the transmission signal, amplifies the signal S7 up to predetermined power, and transmits the signal S7 via the antenna 8. Thus, the transmission signal S7 is transmitted from the transmitter 1 synchronously with the timing of the transmission slots TX.
As shown in FIG. 3, a receiver 10 comprises an antenna 11, a reception circuit 12, a transmission line estimation circuit 13, a demodulation circuit 14, a slot connection circuit 15, a deinterleaving buffer 16, and a Viterbi decoding circuit 17. The receiver 10 receives the transmission signal S7 transmitted from the transmitter 1 via the antenna 11 and inputs the signal S7 to the reception circuit 12 as a reception signal S11. The reception circuit 12 amplifies the input reception signal S11 and fetches a base band signal by applying frequency conversion to the reception signal S11. Then, the circuit 12 applies filtering to the base band signal, obtains reception symbol groups S12 corresponding to the above transmission symbol groups S6 by applying analog-digital conversion to the base band signal, and outputs the groups S12 to the transmission line estimation circuit 13.
The transmission line estimation circuit 13, which is a circuit for examining characteristics of a transmission line and performing equalization corresponding to the examination results, estimates characteristics of a transmission line by referring to the pilot symbols P in the reception symbol groups S13 and calculates inverse characteristics of the transmission line in accordance with the estimation result. Moreover, the transmission line estimation circuit 13 convolution-multiplies respective information symbol portions of the reception symbol groups S12 in a time domain by the values of the inverse characteristics of the transmission line by using an equivalent circuit comprising an equalizer so as to eliminate such influence as fading caused in the transmission line. According to this processing, the transmission line estimation circuit 13 restores the transmitted information symbol groups S5 and outputs them to the demodulation circuit 14 as the reception information symbol groups S13.
The demodulation circuit 14 restores coded bit groups S14 corresponding to the coded bit groups S4 in the transmission side by applying predetermined demodulation to the reception information symbol groups S13 and outputs the groups S14 to the slot connection circuit 15. In this connection, each bit of the coded bit group S14 is not a binary signal having a value of 0 or 1 but a multi-valued signal due to noise components added in the transmission line. The slot connection circuit 15 is a circuit for connecting the coded bit groups S14 fragmentarily obtained in slots one another to make a continuous signal. The circuit 15 connects the coded bit groups S14 when they are accumulated up to the storage capacity of the deinterleaving buffer 16 at the rear stage and outputs the resulting coded bit series S15 to the deinterleaving buffer 16.
The deinterleaving buffer 16, which has a storage capacity for a plurality of slots, successively stores the supplied coded bit series S15 in its internal storage area, restores the order of the coded bit series S15 to the original order by performing the backward process of the re-sequencing performed in the interleaving buffer 3 of the transmitter 1, and outputs the resulting coded bit series S16 to the Viterbi decoding circuit 17 (hereinafter, the restoring to the original order is referred to as deinterleaving). The Viterbi decoding circuit 17, which comprises a soft decision Viterbi decoding circuit, determines a possible trellis of the convolution code in use based on the input coded bit series S16, estimates a maximum-likelihood state (so-called maximum-likelihood series estimation) out of all state transitions which can be used as data to restore and output transmitted information bit series S18.
In the conventional receiver 10, sent symbols are temporally arranged within each slot. Therefore, influences caused in a transmission line are eliminated by performing convolution multiplication in a time domain by an equivalent circuit comprising an equalizer, resulting in highly complicated structures of the receivers. Moreover, in the aforesaid TDMA system, the communication quality may vary depending on the timing of a transmission slot TX. In the conventional receiver 10, the reliability, which shows the communication quality of a transmission slot TX, is not reflected on any coded bits sent via the slot. Therefore, there are problems that maximum-likelihood series estimation by the Viterbi decoding circuit 17 cannot be accurately performed, and transmitted information bit series cannot be accurately restored.